Mac OS X only: Skitch is the best screenshot app for Macs. It's finally out of a very, very long beta with a 1.0 release, adding some neat sharing/social features for free users and many more goodies for $US14.95 per year.
Why is Skitch the best screenshot app for Macs? Because it makes everything annoying about screen capture, image sizing, annotating and sharing very easy and intuitive. The interface is geared toward dragging, dropping and quickly putting images into other apps, and after a few days, you'll never want to go back to the cumbersome world of File->Save As. It's safe to say that it's a software standard among the Mac-using Lifehacker editors.
In its 1.0 release, Skitch has upped the social and sharing features of its still-free release, making it simple to quickly send out a screenshot over Twitter, Facebook, a copied web link or even your own server or web storage space. All the other features of Skitch that we've come to love are still there, too, though it seems file formats have been somewhat restricted to JPG, PNG and the custom .skitch. Tagging and image sets, too, have been added to the mix for free users, which makes heavy Skitch use scale over longer projects.
For $US14.95 per year, you get a lot of new and handy tools in Skitch Plus, including "full-length" webpage captures, resizing presets, major improvements to your Skitch space on the web, and an entirely ad-free experience. The full list of free/paid features are broken down at Skitch's redesigned homepage.
Skitch is a free download for Mac systems only, with a paid upgrade available.